Electrodialysis (ED), electrodeionization (EDI) and related methods and devices were initially developed during the 1950s, and have since that time been improved to the point that such systems are commonly employed to purify fluids for a variety of applications. In general, ED, EDI, and related methods and devices purify fluids through electric field-mediated transfer of ions through membranes from diluting Streams passing through xe2x80x9cless concentratedxe2x80x9d, ion depeleting compartments to concentrating or brine streams passing through xe2x80x9cmore concentratedxe2x80x9d, ion concentrating compartments. Generally, anion transfer (i.e. cation rejecting) and cation transfer (i.e. anion rejecting) membranes are alternated in ED and EDI methods and devices, the membranes being placed between an anode (positive electrode) and a cathode (negative electrode) across which a DC electric field is applied transverse to the fluid flow directions. Anion transfer membranes allow passage only of low molecular weight negatively charged species (anions), and cation transfer membranes allow passage only of low molecular weight positively charged species (cations). Transfer of ions across membranes is mediated by the attraction of the anions to the positively charged anode and the cations to the negatively charged cathode. The combination of an anode, a cathode, and the alternating anion and cation transfer membranes there between is commonly referred to as an ED or EDI xe2x80x9cstackxe2x80x9d. Such stacks may also include cation or anion transfer membranes alternating with substantially non-ion-selective membranes, that is membranes which are not substantially selective for either anions or cations.
EDI differs from ED in that one or more EDI compartments formed by membranes include ion exchange media. The media, typically in the form of resin fibers, fabrics, beads or granules, is present in diluting compartments and sometimes also in concentrating compartments of an EDI device. An EDI compartment may contain either cation exchange resins, anion exchange resins, or a random or structured combination of cation exchange resins and anion exchange resins. The resins reside in the space between alternating anion and cation transfer membranes. In response to the transverse DC electric field, ions are transferred, for example, from diluting to concentrating compartments via the diluting compartment resins and adjacent membranes. The resins form a conductive bridge for movement of ions associated therewith to the ion exchange membranes and thus out of the diluting compartment. The resin facilitates mass transfer of ions by increasing the area available for mass transfer and by decreasing the distance in solution that the ions must travel in order to be removed from the diluting compartment, thus reducing the electrical resistance of the unit, especially in the diluting compartment. In EDI, as the product becomes more pure, the electric field splits water to hydrogen and hydroxide ions which continuously regenerate the membranes and the resins at least in part. The main advantages of EDI processes include continuous operation; stable product quality; the ability to produce high purity product without requiring periodic chemical regeneration; and reduced amounts of waste products.
One area in which EDT technology is gaining momentum is production of ultrapure makeup water for electric power plants. EDI was initially used in the electric power industry in 1991, and since that time more than 50 EDI devices have been installed in such plants. In these plants, EDI has partially or completely replaced the prior conventional (i.e. chemically regenerated) ion exchange resin beds, resulting in substantial operating cost savings. For example, ion exchange units are frequently used to purify blowdown (waste water) for recycling, requiring frequent regeneration, consuming large volumes of acid and caustic, and necessitating constant maintenance. Such exhaustion (degeneration) and regeneration can also result in variations in demineralization performance, thus affecting reliability of use. With the advent of EDI systems, deionization and regeneration are simultaneous and continuous, and problems associated with periodic regeneration are no longer present.
EDI is highly efficient in removing a substantial variety of ions from water. Strongly ionized substances such as sodium, calcium, magnesium, chloride, fluoride and sulfate are examples of ions which are routinely substantially completely removed from water using multi-step purification systems which include one or more EDI units. Weakly ionizable species such as CO2, silica, boric acid and ammonia may also be removed using EDI. Similarly, ethanolamine (ETA) and methoxypropylamine (MPA) are also readily removed by EDI. However, complete removal of non-ionized and non-ionizable organic substances such as ethanol and glyoxal, is not as easily accomplished.
U.S. Pat. No. 5,116,509 discloses use of an ultraviolet (UV) treatment step for deionized tap water prior to EDI treatment, but fails to teach or to suggest a system which processes such tap water water initially containing non-ionized or non-ionizable carbon compounds in addition to ionic or ionizable organic species. xe2x80x9cSubstantially complete removalxe2x80x9d of total organic carbon (TOC) is purported to have been obtained using the system of U.S. Pat. No. 5,116,509. The patent teaches that following an initial deionization, organic species may be added to the deionized tap water and applied UV may break down the added organic species into smaller molecules, some of which are ionic and/or ionizable, allowing subsequent EDI to achieve the claimed TOC reduction. When some part of the added organic species is already in ionic and/or ionizable form, the claims for such a process may not be totally true, however. Although UV may convert non-ionizable organic species into ionic and/or ionizable organic species, UV may also convert ionic and/or ionizable organic species into non-ionizable organic species. If a significant amount of the organic species is in ionic and/or ionizable forms, application of UV may increase the concentration of non-ionizable organic species in the subsequent EDI feed, and may result in a lower organic carbon reduction than without UV. Further, the ionic or ionizable organic species may absorb UV further reducing UV available for converting non-ionizable organic species into ionic or ionizable organic species.
A need remains, therefore, for additional methods and devices capable of obtaining improved purity product from purification systems. In particular, additional methods and devices are needed which can remove non-ionized or non-ionizable carbon compounds in multi-step purification systems.
The invention is directed to methods and devices for removing substantially all inorganic and organic carbon compounds from water. In accordance with the present invention, one or more first deionization stages remove ionic and/or ionizable contaminants. The product stream of such first deionization stages is exposed to an organic carbon bond-breaking agent prior to becoming the feed stream of one or more second deionization stages. Such exposure causes non-ionized organic carbon compounds in the first deionization product stream to ionize or become ionizable, facilitating removal in such second deionization stages. Further, since ionic and/or ionizable contaminants are substantially removed in the first deionization stages, the amount of bond breaking agent required to convert the non-ionizable organic carbon compounds to ionic or ionizable organic carbon compounds is minimized.
In accordance with the invention, such first one or more deionization stages remove the ionic and/or ionizable inorganic and organic carbon species. The effluent from such first deionization stages is exposed to organic carbon bond-breaking agents, such as UV (preferably 184.9 nm wavelength or less) including catalyzed UV and/or other oxidizing agents (e.g., oxygen, ozone, singlet oxygen, hydrogen peroxide, hydroxide radical, means to produce singlet oxygen or hydroxide radical or combinations thereof). These agents break down organic carbon compounds from such first deionization stages. With the subject invention, it is not necessary to provide a level of UV energy sufficient to break down organic components all the way to H2O and CO2, but rather it is sufficient to break down those components into compounds which may be removed in subsequent removal stage(s).
In one embodiment, the invention provides a method of removing inorganic and organic carbon contaminants from water. The method involves the steps of: flowing such water through one or more first removal units to produce a first product stream having less organic carbon compounds; exposing such first product stream for a predetermined time to an organic carbon bond-breaking agent (preferably at a predetermined temperature) sufficient to produce a second product stream containing at least a portion of the remaining organic carbon compounds in an ionized or ionizable form; and flowing such second product stream containing such organic carbon compounds in ionized or ionizable form through one or more second removal units, wherein such organic carbon compounds in ionized or ionizable form are substantially removed. Such removal units may be independently selected from reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), electrodeionization (EDI), filled cell electrodialysis, and electrodiaresis devices; chemically or electrically regenerable ion exchange (IX) systems; and activated carbon and other sorbent beds.
In another embodiment, the invention provides a device for removing organic carbon compounds from water. The device involves: a first electrodeionization (EDI) unit having at least one diluting compartment extending in fluid communication between a first feed stream inlet and a first product stream outlet; a second EDI unit having at least one diluting compartment extending in fluid communication between a second feed stream inlet and a second product stream outlet; a fluid flow path coupling the first product stream outlet and the second feed stream inlet; and an ionizing unit for exposing a fluid flowing along such fluid flow path to at least one organic carbon bond-breaking agent thereby providing substantial removal of organic carbon compounds in the second deionization unit.
It will be understood that such second stages for removing ionized and/or ionizable carbon species may in fact be the same device as that used for the first stage for removing such species. For example, the fluid may be contacted with one or more first stages for removing ionized and/or ionizable carbon species, then contacted with such an organic carbon bond-breaking device and subsequently recycled to such first stages for removing ionizable and/or ionized carbon species. Such combination of bond-breaking and removal devices may operate on batch or on feed-and-bleed continuous bases.
In yet another embodiment, the invention provides a method for removing both ionizable and/or ionized carbon compounds, and non-ionized and/or non-ionizable carbon compounds from water. The method involves the steps of (a) processing a first stream of the water with a removal apparatus for removing ionized and/or ionizable carbon compounds from the water to produce a first product stream; (b) contacting the first product stream with an agent for converting non-ionized and/or non-ionizable carbon compounds into ionized and/or ionizable carbon compounds to form a second product stream; (c) processing the second product stream with a second removal apparatus for removing ionized and/or ionizable carbon compounds from the water to form a third product stream; and (d) recovering the third product stream from step (c). In alternative embodiments, the method includes the steps of recovering the first product stream from step (a) and recovering the second product stream form step (b).
In still another embodiment, the invention provides a device for removing both ionizable and/or ionized carbon compounds, and non-ionized and/or non-ionizable carbon compounds from water. The device involves (a) a first removal means for removing ionized and/or ionizable carbon compounds from the water to produce a first product stream; (b) a conversion means for converting non-ionized and/or non-ionizable carbon compounds in the first product stream into ionized and/or ionizable carbon compounds to form a second product stream; (copyright) a second removal means for removing ionized and/or ionizable carbon compounds from the second product stream to form a third product stream; and (d) a recovery means for recovering the third product stream. In alternative embodiments, the device also includes a second recovery means for recovering the first product stream and a third recovery means for recovering the second product stream.
It will also be understood that xe2x80x9corganic carbon in an ionized or ionizable formxe2x80x9d includes carbon dioxide, bicarbonate and/or carbonate which results from the bond-breaking of an organic carbon species.
As used herein, the term xe2x80x9cionizablexe2x80x9d as in, for example, organic carbon in an ionizable form, refers to organic carbon which can be ionized by proximity to the surface of ion exchange media.